1. Field of the Invention
The present invention is in the field of compositions for rewritable optomagnetic recording media.
2. Description of the Prior Art
In rewritable optomagnetic recording media for recording and reading information under the irradiation of light, a magnetic membrane for optomagnetic recording has been formed with a thin film system composed of the combination of an amorphous rare earth and a transition metal (hereinafter referred to as the RE-TM system).
In the case where the optomagnetic recording with the RE-TM system is carried out using ferromagnetic vertical magnetization of the RE-TM system, the recording is effected only in the amorphous state of the magnetic membrane and the ferromagnetic property is eliminated in the crystalline state where the recorded information is lost or recording of the information becomes impossible. Accordingly, it is desirable in the RE-TM system utilizing a magnetic membrane for optomagnetic recording that the phase transition temperature, Tcry, be as high as possible. The phase transition temperature of the RE-TM system is about 240.degree. C. in the systems Tb-FeCo, Gd-FeCo and Dy-FeCo.
On the other hand, where the information recording or erasing is carried out by a so-called Curie point recording, it must be carried out at an elevated temperature near the Curie point of the RE-TM magnetic membrane which is about 100.degree. to 300.degree. C. Accordingly, if the phase transition temperature is low, the phase transition occurs on recording and erasure resulting in degradation of the magnetic properties or elimination of the ferromagnetic property. Further, since thermal diffusion of the constituent elements occurs even at room temperature in the amorphous RE-TM system magnetic membrane, degradation in the magnetic property results during long time storage even at room temperature in the case of a low phase transition temperature.
Consequently, development of an RE-TM magnetic material having a sufficiently high phase transition temperature to be significantly above the Curie point is desired for an RE-TM magnetic membrane of this type. On the other hand, it is also desirable that the Kerr rotation angle .theta.k be as great as possible in order to obtain a high signal-to-noise (S/N) or carrier to noise (C/N) ratio. It is also desired that the recording bit diameter be smaller in order to improve the recording density. The minimum diameter d for the recording bit is represented by: ##EQU1## where Ms is the saturation magnetization, Hc is the coercive force and Ew is the magnetic wall energy. Thus, high Ms and Hc values are desired for the RE-TM magnetic material in a recording medium of this kind. In conventional RE-TM magnetic membranes of the prior art, there has been a problem in that the phase transition temperature has been generally too low.
Japanese Patent Application No. 57-58868, published Oct. 15, 1983, describes an optomagnetic recording medium made of an alloyed layer having uniaxial magnetic anisotropy composed of an amorphous rare earth metal and a transition metal system, the alloyed layer comprising at least one atomic % germanium.
There is also a publication entitled "Magneto-Optic Type Erasable Disk Memory", published in October, 1984 by Sony Corporation which describes a magneto-optical layer of Tb-FeCo.